Packaging electronic devices provides protection to the internal components of the package from the external environment. However, the packaging of sensor devices, such as pressure sensors, presents the packaging engineer with many challenges. Because the sensor must have access to the external environment to perform its sensing function, the package must have an opening, exposing the internal components of the package to that external environment.
In conventional packaging, the integrated circuits are protected from the external environment by a molding or a coating, typically made from epoxy resin. This protective encapsulate ensures that the internal components of the package, including the devices and interconnections, are not exposed to the environment. The device is protected from the effects of moisture and contamination, which can lead to reliability concerns. In many electronic packages with a sensor device, however, epoxy cannot be placed directly onto the sensor device because of the stress induced by thermal expansion mismatches between the epoxy and the sensor device. Induced stresses may cause false readings by the sensor device.
Alternative protective coatings can be found in the form of gels, such as silicone and fluorosilicone. Some existing electronic sensor packages include gel coatings to reduce the thermal expansion mismatches. Gels provide a moisture barrier, preventing liquid water from contacting the surface of the device and, hence, from causing corrosion. Typically, these packages provide an opening in the gel coating over the top of a small portion of the sensing device. The opening enables the sensor device to have access to the external environment whilst retaining protection for the other internal electronic components. However, such gels are permeable to moisture vapor and do not provide the same level of protection for the remaining device as the molding or epoxy encapsulate.
Another existing approach for protecting electronic packages is to place a protective gasket around the sensor and then clamp the gasket within a housing. The gasket seals the sensor device from the rest of the housing, which can be filled with a protective material to encapsulate other electronic devices. Yet another approach is to mount two devices within a surface mount package. The main bulk of the electronics is then molded, whilst the sensor is left open in a cavity. The cavity can then be filled with a gel to provide some level of protection for the sensor.
However, problems exist with the gasket and molded approaches due to their requirement that the sensor device and other electronics must be placed sufficiently far apart from one another to allow for the positioning of the gasket or to allow for sufficient molding material to encase the electronics. This segregation of the internal components takes up valuable space within the package and places size constraints on the package. In addition, since the sensor device cannot be placed in close proximity to the electronics, there can exist problems with the sensor performance. For example, in the case of capacitive sensors, the capacitance of the interconnections between the sensor and the electronics can contribute to the sensor signal, giving inaccurate sensor readings.
Accordingly, there is a need in the art to develop new methods of partitioning electronic packages, particularly for packages with sensor devices.